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IEEE ELECTRON DEVICE LETTERS, VOL. 27, NO. 6, JUNE 2006
High-Performance E-Mode AlGaN/GaN HEMTs T. Palacios, Student Member, IEEE, C.-S. Suh, A. Chakraborty, S. Keller, S. P. DenBaars, Senior Member, IEEE, and U. K. Mishra, Fellow, IEEE
Abstract—Enhancement-mode AlGaN/GaN high electronmobility transistors have been fabricated with a gate length of 160 nm. The use of gate recess combined with a fluorine-based surface treatment under the gate produced devices with a threshold voltage of +0.1 V. The combination of very high transconductance (> 400 mS/mm) and low gate leakage allows unprecedented output current levels in excess of 1.2 A/mm. The small signal performance of these enhancement-mode devices shows a record current cutoff frequency (fT ) of 85 GHz and a power gain cutoff frequency (fmax ) of 150 GHz. Index Terms—Enhancement mode (E-mode), GaN, high electron mobility transistor (HEMT), millimeter-wave frequencies.
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HE UNIQUE properties of Group III-nitride-based semiconductors have fueled their use in many different applications. The wide bandgap range that can be covered with this semiconductor family has allowed the fabrication of light emitting diodes (LEDs) and laser diodes with wavelengths ranging from deep ultraviolet to green [1]. GaN-based white LEDs are already been used in many lighting applications where the high efficiency and lifetime of these devices are needed. Nitride-based semiconductors are even considered for infrared emitters based on intersubband transitions. On the other hand, the excellent piezoelectric properties of GaN and AlN are not present in any other semiconductor and have motivated the use of these materials in surface acoustic wave (SAW) devices [2], [3]. Also, these semiconductors are characterized by outstanding electronic and transport properties. Hall mobility in excess of 2000 cm2 /V · s and carrier densities in excess of 1.2 × 1013 cm−2 are reproducibly obtained in AlGaN/GaN heterostructures, which has allowed very intense research on different kind of nitride-based transistors. In recent years, excellent results have been demonstrated in depletion mode (D-mode) AlGaN/GaN high electron mobility transistors (HEMTs). More than 32 W/mm of output power have been measured at 4 GHz in field-plated HEMTs [4], and we have recently demonstrated AlGaN/GaN HEMTs with output power in excess of 10 W/mm at 40 GHz [5]. By correctly tuning the harmonic frequencies and enhancing the confinement of the two-dimensional electron channel, it is possible to combine very high efficiencies (64%) with an output power as high as 8.4 W/mm at 15 GHz [6]. GaN-based HEMTs have also Manuscript received January 27, 2006; revised March 14, 2006. This work was supported in part by the Center for Advanced Nitride Electronics and the Millimeter-wave Initiative for Nitride Electronics funded by the Office of Naval Research and monitored by Dr. H. Dietrich and Dr. P. Maki. The review of this letter was arranged by Editor T. Mizutani. The authors are with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106-9560 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/LED.2006.874761
demonstrated an excellent high frequency performance with current gain cutoff frequency (fT ) of 163 GHz [7] and a power gain cutoff frequency (fmax ) of 230 GHz [8]. Enhancement mode (E-mode) devices are also attracting a great interest as they allow the fabrication of simpler power amplifier circuits by using a single-polarity voltage supply. On the other hand, in high-power switching applications, they offer the increased safety of a normally off device. Finally, the use of a nitride semiconductor in digital electronics has been limited by the lack of p-channel transistors, preventing low-power complementary logic. The combination of E-mode and D-mode devices in direct-coupled logic could mitigate this problem. However, in spite of the excellent results achieved in D-mode devices, the performance of E-mode devices is still modest. In 1996, Asif Khan et al. [9] reported the first E-mode AlGaN/GaN transistor. This device had a threshold voltage of +50 mV and a maximum current of 30 mA/mm. Lanford et al. [10] used a recess etch under the gate to fabricate E-mode devices with a threshold voltage of VGS = 0.47 V, maximum drain current of 455 mA/mm, and a current-gain cutoff frequency of 10 GHz in devices with a gate length of 1 µm. By using a 10-nm-thick AlGaN cap layer, Endoh et al. [11] have fabricated unrecessed E-mode devices with a gate length of 120 nm. These devices have a threshold voltage of 0 V, and they show a record fT of 58 GHz and fmax of 108 GHz with a maximum drain current of 550 mA/mm for a VGS = 2 V. Cai et al. [12] have used a fluorine-based plasma treatment followed by a high-temperature annealing to fabricate E-mode devices with a maximum drain current density of 310 mA/mm and a peak transconductance (gm ) of 148 mS/mm. Recently, Micovic et al. [13] have significantly improved the performance of E-mode devices by combining a gate recess with an n+ cap layer to reduce access resistances. These devices showed a maximum transconductance of 400 mS/mm and a maximum current of 900 mA/mm. In spite of the important work performed over the last years, all these results are still behind the state-of-the-art depletion-mode AlGaN/GaN HEMT. The performance of E-mode devices has traditionally been limited by their low transconductance and very high parasitic resistances. These two factors, in combination with the low turn-on voltage of the Schottky gates, have limited the maximum drain current of these devices to less than 600 mA/mm in most of the cases. In this letter, we present AlGaN/GaN E-mode HEMTs where we have maximized the transconductance by reducing the access resistances and gate length of the device. Also, the use of short gate lengths significantly reduces the on-resistance below the gate contact and increases the maximum current of the device. Combining these approaches, we have fabricated E-mode devices with a performance similar
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PALACIOS et al.: E-MODE ALGaN/GaN HEMTs
to the one in state-of-the-art D-mode transistors: more than 1.2 A/mm of maximum drain current and excellent high frequency performance (fT = 86 GHz, fmax = 152 GHz). The samples used in this paper were grown by metal–organic chemical vapor deposition on Si-face 4H SiC. The sample structure consists of a 1.8-µm GaN buffer followed by a 1-nm In0.1 Ga0.9 N back-barrier to increase the confinement of the electrons in the channel [8]. An 11-nm-thick GaN channel was grown on top of the InGaN back-barrier. Finally, the sample was capped by a 25-nm Al0.33 Ga0.67 N barrier. To improve the charge density and the electron mobility, a 1-nm-thick AlN layer was grown between the GaN channel and the AlGaN barrier. The electron density and mobility in the channel are 1.3 × 1013 cm−2 and 1721 cm2 /V · s, respectively, as estimated from Hall measurements. During the processing of the transistors, a Ti/Al/Ni/Au multilayer was used for the ohmic contacts and annealed at 870 ◦ C for 30 s. A Cl2 /BCl3 -based plasma dry etch was used for the isolation, and a Six Ny layer was deposited by plasma-enhanced chemical vapor deposition to eliminate current dispersion. Two e-beam lithographies were used during the gate definition. The first lithography defined the foot of the gate. Then, the gate was transferred to the Six Ny layer with a CF4 /O2 /CHF3 -based dry etch. A 3-min overetch at 100 V was performed to assure the complete removal of the Six Ny below the gate and to partially deplete the channel underneath by introducing fluorine ions into the AlGaN barrier, in a process similar to the one described by Cai et al. [12]. It was also found that the plasma treatment significantly increases the turn-on voltage of the gates. A 12-nm gate recess was performed with BCl3 /Cl2 -based plasma, aiming for a gate-to-channel distance of 13 nm. Finally, a second e-beam lithography defined the top part of the gate. A Ni/Au/Ni multilayer was deposited for the gate contact. The gate length of all the devices was 160 nm while the gate width was 2 × 75 µm. The source-to-gate distance was 0.6 µm and the gate-to-drain distance was 0.9 µm. From the transmission-length measurements (TLM), we have estimated an ohmic-contact resistance of 0.5 Ω · mm and a sheet resistance of 380 Ω/sq. The dc characteristics of the devices were measured with an Agilent 4155 B semiconductor parameter analyzer, while an Agilent PNA E8361A network analyzer was used to characterize the small signal performance of the transistors up to 67 GHz. To evaluate the dispersion in the devices, 200-ns pulsed current–voltage (I–V ) measurements were used. As seen in Fig. 1, these devices show an unprecedented high drain output current of 1.2 A/mm. There are different factors that contribute to this high value. First, the gate of the devices could be biased to +4 V without excessive gate leakage (IG = 0.7 mA/mm at VGS = 1 V; IG = 2.4 mA/mm at VGS = 3 V; and 12 mA/mm at VGS = 4 V when VDS = 5 V). We believe that the reason for the high turn-on gate voltage is the fluorine surface treatment that occurs during the Six Ny overetch process and may modify the band diagram in the AlGaN cap layer by introducing fixed charges. Further studies to clarify this effect are being carried out and will be published in the future. The second reason for the high drain current is the very high transconductance (gm ) of these devices. As shown in Fig. 2, gm exceeds 400 mS/mm as a result of the
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Fig. 1. I–V output characteristics of E-mode AlGaN/GaN HEMT.
Fig. 2. Transfer characteristics of E-mode devices. To calculate the threshold voltage, the IDS curve was linearly extrapolated at its maximum first-derivative (slope) point (i.e., the point of maximum gm ). The intercept of that extrapolation with the VGS axis gives the threshold voltage. As a reference, the transfer characteristics of a D-mode transistor are also plotted.
combination of low access resistances, reduced gate-to-channel distance, and high electric fields of a short gate length. Finally, the very short gate length allows a low value for the parasitic on-resistance under the gate (Ri ), which significantly degrades the output current in E-mode devices with longer gates. In spite of the high charge density in the access region and the very short gate length, the three terminal breakdown voltages of these devices were always in excess of 55 V. From the gm versus VGS curve in Fig. 2, a threshold voltage of +0.1 V can be calculated, which confirms the E-mode character of these devices at VDS of 5 V. Due to short channel effects, the threshold voltage becomes more negative as the drain voltage increases. At VDS = 10 V, the threshold voltage is 0 V, while at VDS = 15 V the threshold voltage becomes −0.2 V. These devices also showed a record of high frequency performance for E-mode devices. At VDS = 10 V, a maximum fT of 86 GHz and fmax of 152 GHz were measured for a VGS voltage of +1.5 V (Fig. 3). From these small signal s-parameter measurements, the variation of the gate-to-source capacitance (CGS ) was extracted. CGS was used to calculate the dependence of the charge density under the gate with the gate voltage (Fig. 4). A total charge density of 1.2 × 1013 cm−2 was
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HEMTs. Although more work has to be done to increase the threshold voltage of these devices, these results confirm the high potential of this kind of AlGaN/GaN transistors for digital electronics based on depletion/enhancement-mode logic and other applications. R EFERENCES
Fig. 3. Typical variation of fT and fmax with gate voltage in AlGaN/GaN E-mode and D-mode devices. The gate length is 160 nm.
Fig. 4. Gate-voltage dependence of the carrier concentration under the gate in an E-mode device with a gate length of 160 nm. The carrier concentration was extracted at each voltage from the CGS obtained from small signal s-parameter measurements following the method described in [14]. The decrease in CGS at high gate voltages may be due to the effect of gate leakage or increase in the source access resistance as described in [15].
estimated at VGS = +2.5 V, while the charge density was only 9.2 × 1011 cm−2 at VGS = 0 V. No dispersion was observed under 200-ns pulsed I−V measurements performed with a load line of 160 Ω and a maximum drain voltage of 20 V. The gate voltage was swept from −3 to +3 V. The combination of the low dispersion with the high drain current and breakdown voltage of these devices promises an excellent large signal performance. In conclusion, E-mode devices with a threshold voltage of +0.1 V have been fabricated on recessed AlGaN/GaN structures. Due to the ultrashort gate length, low parasitic resistances and the high gate turn-on voltage, a record current of 1.2 A/mm has been achieved. These devices also show an excellent high frequency performance similar to the state-of-the-art D-mode
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